1. Field of the Invention
The invention relates to drawing optical fibers from a glass preform. In particular, the invention is directed to methods and apparatus for forming a melt zone in a portion of the preform.
2. Description of the Prior Art
In the prior art it is well known to use lasers to form a heat zone to heat and melt a portion of a glass preform in order to draw an optical fiber therefrom. Since the laser beam is easily focused and directed by ordinary geometric optics, there are several ways in which a laser might be employed to heat glass. One or more laser beams may simply be directed at spaced sections of the surface of the glass preform, or, using beam splitting techniques, the laser beam may be made incident on the surface of the preform at a plurality of diametrically opposed points as shown in U.S. Pat. No. 4,012,213 which issued on Mar. 15, 1977 to Haggerty et al. However, to ensure uniform heating of the preform, it is preferred that the beam be made incident over substantially the entire periphery of the preform in the zone to be melted. This may be done by forming a beam having an annular cross section and directing such a patterned beam along an axis that is colinear with the axis of the fiber as it is drawn. U.S. Pat. No. 3,865,564 to Jaeger et al., which is assigned to Bell Telephone Laboratories, Incorporated and issued on Feb. 11, 1975, describes such a laser drawing apparatus.
The Jaeger et al. patent discloses a laser beam being directed through an eccentrically mounted rotatable transparent convex lens. Such a configuration focusses the incident beam at the focal point of the lens, but off the axis of rotation. By rotating the lens at a sufficiently high velocity, a patterned beam is produced which effectively has an annular cross section. The annular beam is then directed towards a frustoconical reflector which has a solid cylindrical glass preform axially disposed therein. The beam is reflected radially inward from the reflector, onto the glass preform, to heat the preform and form a melt zone from which the glass fiber is drawn. This system has been found to be most satisfactory for drawing fibers using a laser, however, the length of the heat zone so formed is relatively narrow (approximately the diameter of the laser beam). The narrow heat zone limits the amount of laser power that can be applied before vaporization of the preform material occurs, which, in turn, limits the diameter of the preform which can be melted.